This invention relates to a novel process for the continuous preparation of perylenetetracarboxylic diimides by fusion of naphthalimide in a mixture of an alkali metal hydroxide and, optionally, an alkali metal carboxylate and subsequent air oxidation. The perylenetetracarboxylic diimides thus prepared are useful as pigments for coatings (for example, in automotive, industrial, and architectural applications) and plastics, as well as for fibers. In general, the process of the invention should give products that are useful wherever corresponding products made from the batch processes are used.
It has long been known that perylenetetracarboxylic diimides can be prepared by fusing naphthalenedicarboxylic diimides in the presence of an alkali or certain amine bases. For example, U.S. Pat. No. 1,200,848 discloses a process in which N-substituted 1,8-naphthalenedicarboxylic diimides are heated in an alkaline melt. It has been noted, however, that this method produces product contaminated with large quantities of impurities that are difficult to remove. See U.S. Pat. No. 1,913,052. The desirability of using commercially more viable methods has spurred efforts to improve fusion processes based on alkali metal bases.
It is possible to improve the quality of the diimides by carrying out caustic fusion in the presence of special additional reagents and solvents, such as special glycol solvents and their monoalkyl ethers. E.g., U.S. Pat. Nos. 3,615,800 and 3,446,810. However, the use of solvents or other special reagents add to the cost.
More elegant processes involving the use of alkali metal hydroxides, optionally in the presence of sodium acetate, have been reported. See Chemical Abstracts, 42:5892i (1948); European Patent Application 54,806; and U.S. Pat. No. 4,588,814. In these methods, the caustic fusion produces an intermediate leuko material that is then oxidized in air (see, e.g., BIOS Final Report, 1484, page 21) or with oxidizing agents such as peroxydisulfates, nitrates, chlorates, adducts of hydrogen peroxide and borates, or hypochlorites (see, e.g., European Patent Application 54,806) to form the perylenetetracarboxylic diimide product.
Although the more recently reported batchwise methods discussed above produce good quality product, a more efficient continuous process would be desirable. For example, the need for an evacuated reactor in the process disclosed in U.S. Pat. No. 4,588,814 would present practical difficulties for a continuous process. Continuous process methods have been reported for other types of pigments, particularly copper phthalocyanine, but until now have not been reported for the preparation of perylenetetracarboxylic diimides. The present invention provides such a continuous process for preparing perylenetetracarboxylic diimides using smaller quantities of alkali than used in the previously known methods, even when such smaller quantities of alkali lead to high viscosity. In addition, increased yields of high quality product can be obtained using the present method.